The invention relates to a method of timing an interfrequency handover in a cellular radio system. The invention relates particularly to a system where data transmission between a terminal equipment and a base station employs several adjacent alternative frequency bands, and which system comprises several different base stations the coverage areas of which overlap at least partly, the base stations utilizing frequency band groups that differ from one another and that are situated next to one another in the frequency range.
In radio systems a signal to be transmitted has to be usually modulated for data transmission on a transmission channel. The modulation is generally carried out by digital modulation methods which are used to transmit a desired signal on a given frequency band. Digital modulation methods where the envelope is not constant are often used. With such modulation methods, if the transmitter is not linear there will be interference outside the frequency band allocated for signal transmission, such interference being called adjacent channel interference. This interference is mainly due to nonlinearities of the transmitter. Nonlinearity of the transmitter is closely related to end amplifier power efficiency of the transmitter. Linear amplifiers cause only a little interference to adjacent frequency bands, but the power efficiency of the amplifiers is low. With nonlinear amplifiers there will be more interference, whereas the capacity will be better.
When cellular radio systems are designed, the purpose is to utilize a frequency band allocated to a system as efficiently as possible. This means that there will be adjacent channel interference in actual radio networks. It is characteristic of a terminal equipment in a radio system that the receiver of the equipment must be able to attenuate even strong signals on an adjacent frequency band. However, adjacent channel attenuation (selectivity) by the receiver is always limited since too great demands lead either to distortion of the desired signal received by the terminal equipment or to excessive increase of power consumption of the terminal equipment. It is characteristic of radio systems that a certain adjacent channel attenuation value is determined for terminal equipment receivers of the system, and the receivers should attain this value. Therefore each terminal equipment knows its own ability to attenuate an adjacent channel signal.
In some situations adjacent channel interference increases to such an extent that the connection may be disconnected. This is called blocking of a receiver. In such a situation, it is important that an interfrequency handover can be performed rapidly and at a correct moment. Examine the situation shown in FIG. 1. A terminal equipment MS communicates with a base station BTS1. Frequency F1 is used over the connection in the downlink direction (BTSxe2x86x92MS). However, the terminal equipment is situated close to another base station BTS2, which transmits to its own terminal equipments on frequency F3. If frequencies F1 and F3 are adjacent frequency bands on the frequency range, the transmission of the other base station BTS2 appears to the MS as adjacent channel interference, since the receiver selectivity is not ideal. Assume that BTS1 and BTS2 are, for example, base stations of different network operators, in which case the MS cannot perform a handover to BTS2. As the interference becomes stronger, there is a risk of the connection from the MS to BTS1 being disconnected.
In prior art arrangements in CDMA systems, a terminal equipment measures the strength of the signals it has received from the base stations. The purpose of the measurements is to search for handover candidates. However, these methods are not applicable in the aforementioned situation, where the other base station is located on another frequency to which the terminal equipment cannot perform a handover. In some arrangements, the interference caused by the terminal equipment""s own transmission in the uplink direction (MSxe2x86x92BTS) to terminal equipments of another operator is estimated on the basis of signal strengths measured from base station transmissions, and this is used as a basis for a handover. However, this method does not take into account the interference to the terminal equipment""s own connection, which means that the handover is not carried out in the best possible manner for the terminal equipment since the call may be blocked before the handover is completed.
The purpose of the invention is to provide a method and an equipment implementing the method such that the aforementioned problems can be solved. This is achieved with a method according to the invention of timing an interfrequency handover in a WCDMA cellular radio system, where data transmission between a terminal equipment and a base station employs several adjacent alternative frequency bands, and which comprises several different base stations the coverage areas of which overlap at least partly, the base stations utilizing frequency band groups that differ from one another and that are situated next to one another in the frequency range, in which system a terminal equipment communicates with a first base station on a first frequency band and measures the strength of a first signal it has received from the base station, and wherein the terminal equipment measures the strength of a second signal transmitted by a second base station on a second, adjacent frequency band. In the method according to the invention, the terminal equipment compares the measurement results of the first and the second signal, and if the second signal exceeds the first signal by a given threshold value, the terminal equipment signals a handover request to the first base station.
The invention also relates to a cellular radio system which utilizes the WCDMA method, and where data transmission between a terminal equipment and a base station employs several adjacent alternative frequency bands, and which comprises several different base stations the coverage areas of which overlap at least partly, the base stations being arranged to utilize frequency band groups that differ from one another and that are situated next to one another in the frequency range, in which system a terminal equipment is arranged to communicate with a first base station on a first frequency band and to measure the strength of a first signal it has received from the base station and to measure the strength of a second signal transmitted by a second base station on a second, adjacent frequency band. In the system according to the invention, the terminal equipment of the system comprises means for comparing the measurement results of the first and the second signal, and means for deciding on the transmission of a handover request to the first base station if the second signal exceeds the first signal by a given threshold value.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the discovery that before the transmission of a terminal equipment interferes with the transmissions of other terminal equipments situated in the area of another base station, the terminal equipment""s own connection may be disconnected when the adjacent channel interference caused by the transmission of the other base station exceeds the adjacent channel selectivity of the terminal equipment receiver.
The method and the system according to the invention provide several advantages. The moment of handover can be selected optimally by monitoring and comparing the measured signal strengths. Further, in the arrangement according to the invention it is possible to adjust the capacity used by the terminal equipment for measurements. Thus, the interference caused by the measurements to the terminal equipment""s own connection can be minimized.